Transmission problem

Hi its me again lol. I resently bought a used
transmission and changed it. It rode
perffect for three weeks then on the way
home from work. The whole car jumped
foward, then it acted like it was out of gear
then it started going again and it starting
making a slamming noise and started
jumping forward again. So pulled over and
checked the fluids and the axles but
everything looked great no leaks and no
seperated axles. So i sit on the side of the
road for my husband to come and let me
remind you i was only a mile from my home.
So my husband gets there and he drives it
three quarters of a mile home and it does it
to him but worse  so we parked it and towed
it home. We got it home and the
transimission is now leaking it goes in to
gear but now it grinds but goes no where in
any gear.  Please help, sorry it was so long i
wanted to give you good detail.